The present invention relates to a laser device for emission of waves in the terahertz range. The present invention can have numerous applications. It can apply especially in the fields of medical imaging, security (detection of metals or gases in airports, etc.). It can also enable quality controls.
Sources of terahertz radiation are sources whereof the output signal has a wavelength in the field of far infrared of the electromagnetic spectrum (300 GHz-10 THz). Sources of terahertz radiation are known, such as carcinotrons (or “BWO” for the English expression “Backward Wave Oscillator”) or the quantum cascade lasers (or “QCL” for the English expression “Quantum Cascade Laser”). A carcinotron is a tube for generating microwaves which operates by prolonged interaction of an electronic beam and an electromagnetic wave.
A quantum cascade laser is a semiconductor laser capable of emitting a light wave using inter subband transitions in hetero-structures with multiples quantum well. This type of laser theoretically produces quantum yields greater than 1 by “recycling” of carriers. The application of an electrical field allows an electron, once it has emitted a first photon in a first well, to passer by effect tunnel to a following well, and so on. One of the disadvantages of existing sources of terahertz radiation is that they are neither compact nor easy to execute outside a laboratory.
Another disadvantage of existing sources of terahertz radiation is that they do not emit radiation covering all frequencies of the terahertz range. In fact, a carcinotron is capable of emitting radiation in frequencies of less than or equal to 1.5 THz. A quantum cascade laser is as such capable of emitting radiation in frequencies greater than or equal to 2.5 THz. Existing sources of terahertz radiation therefore do not emit radiation in a frequency range between 1.5 THZ and 2.5 THz.
An aim of the present invention is to propose a compact laser device. Another aim of the invention is to propose a laser device capable of emitting a wave in a frequency range between 0.5 THz and 5 THz, and preferably between 1.2 and 2.8 THz.
For this purpose the invention proposes a laser device for wave emission in a frequency range in the terahertz range, comprising in combination:                waveguide extending longitudinally according to an axis A-A′, the waveguide comprising a proximal end and a distal end,        a superconducting coil coaxial to the waveguide and arranged at the level of the proximal end of the waveguide,        a p-Ge p-doped germanium crystal arranged inside the coil such that the windings of the superconducting coil at least partially enclose the p-Ge crystal,        cooling means containing coolant in the liquid state, the superconducting coil and the p-Ge crystal being arranged in the cooling means and the waveguide extending partially to the exterior of the cooling means, means for eliminating coolant in the liquid state in the waveguide, said means comprising two windows transparent to the light radiation in the terahertz range at the level of the proximal and distal ends of the waveguide.        
Preferred, though non-limiting, aspects of the device according to the invention are the following:                the means for eliminating coolant in the liquid state in the waveguide replace it with coolant in the gaseous state,        the means for eliminating coolant in the liquid state in the waveguide also comprise a heating element in thermal contact with the waveguide for avoiding coolant condensation,        the means for eliminating coolant in the liquid state in the waveguide comprise a vacuum made in the waveguide,        the windows are made of material transparent in the terahertz range tel such as crystalline quartz, Mylar, Teflon®, crystalline ZnSe, sapphire, high-purity silicon or other high-purity semiconductive crystals,        the window transparent to light radiation positioned at the level of the distal end of the waveguide is a lens,        the device also comprises a resonator comprising at least two mirrors deposited on two respective plates of high-purity germanium crystal in contact with the p-Ge crystal, the p-Ge crystal being arranged between the crystal germanium plates according to the axis A-A′ of the waveguide, the mirrors are made of material selected from silicon dioxide SiO2 and high-purity germanium,        the mirror the farthest from the waveguide is spherical,        the device also comprises a convergent lens arranged inside the waveguide such that:                    the distance between said lens and the resonator is the greatest possible, and            the light beam originating from the resonator does not come into contact with the internal walls of the waveguide,                        the focal distance of the convergent lens is selected such that the light beams exiting from said convergent lens do not come into contact with the walls of the waveguide,        the waveguide comprises a divergent cone trunk and a tube coaxial to the divergent cone trunk, the tube extending at the level of the large base of the cone trunk, the small base of the divergent cone trunk forming the first end of the waveguide,        the superconducting coil comprises at least two layers of windings, each winding of a new layer being positioned in a hollow formed by two adjacent windings of the preceding layer.        